It's lines from Shelley's sonnet Ozymandias, and to make sense of it you need to put them in context. Here's the whole poem, with your lines in bold:

I met a traveller from an antique landWho said: Two vast and trunkless legs of stoneStand in the desert. Near them on the sand,Half sunk, a shatter'd visage lies, whose frownAnd wrinkled lip and sneer of cold commandTell that its sculptor well those passions readWhich yet survive, stamp'd on these lifeless things,The hand that mock'd them and the heart that fed.And on the pedestal these words appear:"My name is Ozymandias, king of kings:Look on my works, ye Mighty, and despair!"Nothing beside remains:round the decayOf that colossal wreck, boundless and bare,The lone and level sands stretch far away.

So, your first line is part of the words engraved on a pedestal, boasting about the great works of the king Ozymandias. But the boast is ironic, because the works have been destroyed by time: only two huge fight and the shattered face of what once must have been a statue remain. "Nothing beside remains", our narrator tells us - there's nothing else left.

And, round the decay of that huge wrecked statue, all there is is sand - metaphorically representing the sands of time, which has brought the mighty (in this sense, quite literally) to its knees.

Shelley, in these lines, is pointing out how the pride, egotism, and powerful feeling of immortality possessed by this ruler has turned to dust, and how "nothing beside remains" of his majestic kingdom and royalty. The monument to this leader sits in the middle of a desert, abandoned, forgotten, eroding away. However, during the king's time period, he probably felt that he was immortal and all-powerful, and hence had his statue inscribed with the ominious "Look on my works, ye mighty, and despair!" logo that was meant to emphasize his power.

But now the traveller is looking on all that is left of the ruler's works, and doesn't feel despair; rather, he feels pity, or nothing, as all that is left of the king's kingdom is the desert, "boundless and bare/The lone and level sands stretch far away".

Also, the line suggests that literary art lasts longer than plastic art. Ironically, the only reason we know of Ozy's great boast is that the poet writes down the traveler’s story. So as the line mocks the ruler, the ruler's voice, and the ruler's pride, the line praises the poet, the poet's voice, and the poet's glory. Shelley's "Ozymandias" reminds us of Shelly's belief that the poet, not the politician, "is the unacknowledged legislator of the world."